In the context of road reconstruction by milling and the removal of mineral deposits by means of surface miners (also by milling), the earth working tools that are used, and in particular the milling bits, are subject to a continuous wear process. Replacement is advisable once the tools reach a specific wear state, since otherwise the ongoing process loses efficiency.
The wear state of the bits and bit holders can be assessed by visual inspection by the machine driver. The wear state of the bit holders is usually assessed by way of so-called wear markings. The wear state of the bits is assessed by way of the longitudinal wear and the rotational symmetry of the wear pattern.
Inspecting the wear state of bits and holders is very time-intensive, and is unproductive since nothing can be produced during that time. The overall process is disrupted and the availability of the earth working machine is thus additionally decreased. It is therefore advantageous if the expected inspection intervals are very long or are optimized in terms of the working sequence, and if the required frequency of inspections is therefore low and they occur at times favorable for operation of the machine.
Replacement of the earth working tools, i.e. of the bits and optionally of the associated bit holders, also requires a large expenditure of time during which the earth working machine cannot be used. Costs are additionally incurred for the tools to be replaced. Maintenance of the earth working tools thus represents a large cost factor for operation of the earth working machine.
The wear on the earth working tools is influenced substantially by the material properties of the substrate to be worked, and by the machine parameters with which the earth working machine is operated. A machine driver will attempt to set the machine parameters in such a way that economic specifications, for example working a specific area within a specified time or in the shortest possible time, are complied with, without stressing the earth working machine more than necessary. This setting is made subjectively, however, and depends substantially on the experience of the respective machine driver. Machine parameter settings that yield good working output, but do not sufficiently consider wear on the earth working tools, are often selected. The result of this is that the earth working tools wear, and must be replaced, prematurely. It can also happen that different combinations of machine parameter settings result in comparable working output values but in greatly different wear behavior.
The material properties of the substrate to be worked can change during the working process. In mining, for example, it can happen that while traveling over a deposit of material to be removed, the hardness of the raw material suddenly rises (“hard spot”). Increased wear then occurs on the tools.
DE 10 2008 045 470 A1 (U.S. Pat. No. 8,386,196) discloses a method for quantitative determination of wear on earth working tools (bits and bit holders) of a milling machine. Here the position in space of at least one point on the earth working tool is sensed. This measurement result is then compared with a reference value, so that the wear on the tool can be sensed quantitatively. A measurement system preferably operating in noncontact fashion, which operates reliably under the harsh environmental conditions in the region of the milling drum and with which the automated wear determination can be accomplished, is provided in order to carry out the method. The method enables automated recognition of the wear state of the earth working tools so that they can be replaced when the wear limit is reached, but without limiting the wear rate itself.
DE 10 2014 015 661 A1 (U.S. Pat. No. 9,103,079) describes a milling machine for earth working, for example for road construction. The mobile milling machine comprises a rotor having milling tools mounted thereon. The working height of the rotor and thus the milling depth, the rotor rotation speed, and the travel speed are adjustable. The spacing of a dimensioning mechanism with respect to the rotor can furthermore be adjusted, and the degree to which the milled material is crushed can thereby be influenced. Associated with the milling machine are various sensors for determining ground characteristics (e.g. density, material thickness, or detection of objects present beneath the ground), travel speed, rotor working height, rotor rotation speed, or the orientation of the dimensioning mechanism. Based on the sensor signals the milling machine can be controlled in such a way that a collision between the rotor and objects present beneath the ground, and thus damage to the rotor, is avoided. The milling machine can furthermore be controlled, as a function of soil density or material thickness, in such a way that a desired mixing ratio of the removed material is achieved. The speed of the milling machine and the rotation speed of the rotor are set as a function of the ground characteristics in such a way that the material is cut most efficiently to the necessary grading or granulometry. In the case of a high material density, for example, the travel speed and/or the rotor rotation speed can be reduced in order to achieve the necessary grading. It is disadvantageous in this context that wear on the rotor or on the milling tools mounted thereon is not considered in the setting of machine parameters. Machine parameters that result in highly efficient cutting performance can thus lead to severe wear with correspondingly short replacement intervals and the high costs associated therewith.